Many physical and chemical processes in a fluid or multiphase reaction mixture comprising a fluid, are suitably carried out in a channel, such as in a tube reactor. Often two conflicting requirements exist: while high velocity is desirable, for example, to obtain good mixing and/or suspension of solid particles in the liquid, a low velocity is desirable for a sufficient long residence time of the mixture in the channel.
A high velocity is, for example, desirable for the formation of Dean vortices in a liquid flowing in a curved channel. Dean vortices are secondary, radial flow patterns that can occur when a liquid is flowed through a curved channel, such as a helical channel, due to centripetal forces.
The favourable effects of Dean vortices on various chemical processes, such as crystallisation processes, are described in WO-A-2006/025741 and WO-A-2009/151322. The favourable effects of Dean vortices, in particular for crystallisation processes, include improved stability of multiphase flows, higher mass and heat transfer, narrow residence time distribution, reduced energy consumption and an increased Reynolds number (Re) at which turbulence sets on. For crystallisation, this results in better control of particle morphology and particle size, narrow particle size distribution and good scalability of the process.
Without wishing to be bound by way of theory, it is believed that these favourable effects of Dean vortices are the result of good radial mixing, in combination with low mixing in axial direction, which results in the flow of the fluid through the cannel becoming more or less plug flow. This flow pattern is believed to result in reduced collusion of, for instance, particles and/or droplets in a liquid with each other, providing reduced attrition and coalescence and a more uniform product.
However, in practice, the problem has been encountered, that either the residence time of the fluid in the channel is very short or the channel very long, due to the minimum fluid velocity for the formation of Dean vortices. A long channel length has the disadvantage that the reactor becomes more expensive and that the energy consumption is higher. If the residence time is too short, the chemical process may be not complete.
Therefore, a need exists for an improved method for carrying out a physical or chemical process, combining the effect of Dean vortices with a short channel and a long residence time of the fluid in the channel.
A process and apparatus for reactive or anti-solvent crystallisation in a curved channel the presence of Dean vortices is described in WO-A-2006/025741. The channel is preferably a helical tube reactor, or spiraled crystalliser, with preferably up to 1000 turns. In the example, the channel length was 3 m, the number of turns 33, the flow rate 100 ml/min, Re was 707, and the Dean number was 272. A disadvantage is that the residence time was only 2.7 s, which is for many applications too short. For example, cooling crystallisation generally requires a much longer residence time.
WO-A-2009/151322 discloses a process and apparatus for carrying out multiphase reactions in the presence of Dean vortices formed due to flow of the feed through a curved channel, such as in the form of a spiral. It is mentioned that preferably a non-pulsating pump is used and that using pulsating pumps such as pistons or membrane pumps result in a fluctuating flow rate having a negative effect on the formation of Dean vortices. Pulsating pumps provide a pulsed flow that is fluctuating between a certain minimum, e.g. zero flow between two pulses and a certain maximum flow during a pulse in one axial direction. Flow reversal due to the pulsating flow is not disclosed. In an embodiment a reactor comprising helical modules is used in a batch process in a closed system. The beginning and the end of the reactor are alternately provided with overpressure and the flow is in this way reversed. According to the description, the length of one-way flow should be maximised and the flow should be reversed only after the mixture has passed a minimum of 25 windings.
GB-A-935 431 discloses oscillatory flow in a tube. A first flow is continuously subjected to a superimposed longitudinal oscillatory movement of sufficient power to repeatedly reverse the flow and produce turbulent flow conditions throughout the tube. Application in curved channels is not disclosed.
U.S. Pat. No. 6,399,031 discloses a curved tubular flow reactor wherein the direction of curvature alternates along the length of the tube. The changes in curvature direction result in a more narrow residence time distribution compared to a helical tube reactor. Reversal in axial flow direction in the channel, is not disclosed. The use of pulsation means, such as pumps, causing the reaction to proceed in a pulsed manner, does not disclose reversing the direction of the flow as pumps are normally provided with check-valves.
US-A-2008/0 316 858 discloses a baffled reactor with oscillatory flow and a channel in the reactor wall and/or a protuberance of the reactor wall, wherein said channel or protuberance is the baffle and is helically in the longitudinal direction. In a helically baffled tube, the baffle is helical, not the tube.
WO-A-2012/095176 describes an apparatus comprising an unbaffled reactor or micro-reactor having a channel with a plurality of directional changes and a hydraulic diameter from 0.1 mm to 10 mm and an oscillating flow device. In use, the net flow of the reaction stream is in the forward direction but the oscillating flow device slows, pauses and/or reverses the forward flow periodically. Piston pumps or diaphragm pumps modified by removing check-valves can be used as pulsators. It is mentioned that if the hydraulic diameter is larger than 10 mm, sufficient turbulence becomes a problem. Crystallisation of oxalic acid and potassium nitrate is described in a reactor constructed from a 10 m PTFE tubing with 2,54 mm internal diameter with continual changes of direction in helical fashion with an inner diameter of the helix of 1.9 cm and a diaphragm pump as oscillator.
US-A-2009/0 304 905 describes a continuous oscillatory baffled reactor with straight parts and connecting U-bend parts and an oscillating bellow. The connecting U-bend parts form only a small part of the length of the reactor.
Zheng et al., Chem. Eng. Sci. 2008, 63, 1788-1799 describe the axial dispersion performance of an oscillatory flow meso-reactor comprising straight tubes connected by U-bend parts. The reactor is curved over only a small part of the length of the reactor. The U-bend parts form only a small part of the length of the reactor.
US-A-2012/0 171 090 describes a continuous tubular flow reactor comprising multiple straight tubes connected by U-bend parts. A pressuring device is used having a simple harmonic motion driving device for providing in the fluid a harmonic reciprocating flow in superposition to the original longitudinal flow. The U-bend parts form only a small part of the length of the reactor.